Keep making flame gems & explode them close to the bottom of the screen. Also, take care of the pesky corners. Set off the star gems on the bottom row to clear rows quicker. It's a rare occurrence, but explode 2 hypercubes which clears everything on the screen giving you more time on the clock.

After you clear all dirt over the line, you can keep swapping gems if you're fast enough. There's no need to be too fast, just get the hang of it.Keep swapping gems all the time even if they don't match so the level doesn't go down. Notice that the clock will still go down. Once you stop, the stage advances down and will award 30 seconds to your clock. The same happens if your clock runs down to zero while doing this but you'll keep playing the next stage. If you clear all the stage you'll get a 90 seconds award. Sometimes this doesn't worth the effort as if there's some indestructible rock it's usually harder to clear everything, and most times the game places new falling gems in a way that it's not easy to reach the bottom with matching jewels. But on early stages in the game it's easy to build a good amount of time.

If you do get two Hypercubes next to each other, before matching them right away, see how many blocks you have left to clear the rows normally. If you only have a couple left and you can clear them without disturbing the Hypercubes, you can get 30 seconds for the normal clear, then immediately match the Hypercubes for the 90 second bonus.